Active sound control systems often require specialized sensors, such as microphones to measure variations in atmospheric pressure. Commercially available electrical microphones, for example, detect pressure applied on their diaphragm by monitoring small changes in the capacitance measured between the diaphragm and a backplate placed closely to the diaphragm. Such electrical microphones are typically of two types: the electret microphone, which is biased by a built-in charge, and the condenser microphone, which is biased by an external voltage source. Electret microphones suffer from long-term instability due to their charge decay, whereas condenser microphones need a substantial external bias voltage, which in some applications is not desirable.
Both types of electrical microphones are susceptible to electromagnetic interference (“EMI”), which is strong at low frequencies. For example, in some active control systems, the presence of high voltages required for the controllers introduces substantial EMI, which increases the noise of the electrical microphones. To minimize EMI, electrical microphones often have a preamplifier attached to the microphone head. Lead wires close to the microphone head introduce extra capacitance, which can degrade the capacitance signal of the microphone. The lead capacitance problem is minimized by placing the preamplifier next to the microphone head. However, having the preamplifier next to the microphone head, even though minimizing EMI and lead capacitance noise, makes the microphone heavy, large, and expensive.
Examples of fiber optic microphones include those disclosed in “Theoretical and experimental study of a fiber optic microphone,” Hu et al., J. Acoust. Soc. Am. 91 (5), May 1992, and “High-temperature fiber-optic lever microphone,” Zuckerwar et al, J. Acoust. Soc. Am. 97 (6), June 1995. Hu et al. disclose a fiber optic microphone that includes a metallized Mylar membrane that is stretched and cemented to a fiber optic microphone case. Zuckerwar et al. disclose a fiber optic microphone cartridge that secures a membrane made of nickel 200 foil. Typical fiber optic microphones have a diaphragm diameter of around 0.55 cm, a diaphragm thickness of around 12.5 μm, and a minimum detectable pressure of around 5 mPa/Hz1/2. Fiber optic microphones have typically not replaced electrical microphones in many applications because their performance and/or cost-effectiveness have been inadequate.